The origin of the solvent dependence of fluorescence quantum yields in dipolar merocyanine dyes

Abstract

Fluorophores with high quantum yields are desired for a variety of applications. Optimization of promising chromophores requires an understanding of the non-radiative decay channels that compete with the emission of photons. We synthesized a new derivative of the famous laser dye 4-dicyanomethylen-2-methyl-6-<i>p</i>-dimethylaminostyryl-4<i>H</i>-pyran (<b>DCM</b>), <i>i.e.</i>, merocyanine 4-(dicyanomethylene)-2-<i>tert</i>-butyl-6-[3-(3-butyl-benzothiazol-2-ylidene)1-propenyl]-4<i>H</i>-pyran (<b>DCBT</b>). We measured fluorescence lifetimes and quantum yields in a variety of solvents and found a trend opposite to the energy gap law. This motivated a theoretical investigation into the possible non-radiative decay channels. We propose that a barrier to a conical intersection exists that is very sensitive to the solvent polarity. The conical intersection is characterized by a twisted geometry which allows a subsequent photoisomerization. Transient absorption measurements confirmed the formation of a photoisomer in unpolar solvents, while the measurements of fluorescence quantum yields at low temperature demonstrated the existence of an activation energy barrier.